If you browse the Spatial Audio section of Apple Music, you'll find music that most certainly doesn't have the feature. Full story from the iMore Blog...
Kind of a bummer. I roll through the Spatial category every so often to see any new tracks I like, then put it in a playlist named spatial. I rarely look to see if “Dolby Atmos” shows during playing it.
Obviously a big library to change but I hope they get more done quicker. I’m not all too keen on the big remix that makes the spatial-ized track sound very different. A straight remix to Spatial is +1 for me.